Many high performance power electronics require cooling devices to prevent them from overheating so as to improve reliability and efficiency thereof. One method for cooling such power electronics is by utilizing heat sinks. The heat sinks operate by transferring the heat away from the power electronics thereby maintaining a lower temperature of the power electronics. There are various types of heat sinks known in thermal management fields including air cooled and liquid cooled devices.
Typically, the heat sink is made up electrically conductive material. Therefore, the power electronics may be coupled to the heat sink with a substrate disposed therebetween to avoid generation of short circuit, which can damage the power electronics. The substrate generally includes an electrically isolating and thermal conductive layer, such as a ceramic layer. In order to attach the ceramic layer to the heat sink and the power electronics, the substrate further includes metal, such as copper brazed or bonded to upper and lower surfaces of the ceramic layer to perform surface treatment to the ceramic layer.
The surface treatment process is typically performed at a high temperature, such as 600° C. to 1000° C. Thus, metal patterns and thickness should be controlled to prevent mechanical distortion as the substrate is cooled to room temperature. The cooling may result in mechanical residual stress at the metal to ceramic interfaces due to the difference in coefficient of thermal expansion (CTE) between the metal and the ceramic layer. For example, the ceramic layer includes aluminum nitride ceramic whose CTE is 4 ppm/° C., and the CTE of the copper metal is 17 ppm/° C.
Additionally, the substrate is formed with a plurality of millichannels on the lower metal for a coolant passing through. However, fabricating the microchannels in the bottom metal of the substrate may relieve some of the residual stress due to removal of the metal. Consequently, the substrate may be deformed.
It is desirable to have a planar substrate for bonding the substrate to the heatsink and the power electronics. Particularly, when the non-planar substrate is bonded to the heatsink at a higher temperature about 250° C., the substrate deforms more and the bond to the heatsink can not be achieved.
Therefore, there is a need for new and improved methods for making millichannel substrate, and cooling device and apparatuses using the millichannel substrate.